User Manual
Fig. 3.3-17
Part-winding star-delta starting
Circuit diagram and connections of the motor windings during starting (Y) and in operation (∆)
Ratings of the starter components
With the exception of the star contactor, contactors and motor protective devices have the same
ratings as with the “normal” star-delta circuit (
see Section 3.3.1). The star contactor should be
selected for (0.5 ... 0.58)
· I
e
because of the larger starting current.
Notes
A sufficiently long switching interval for transition from star to delta operation should
be ensured,
in accordance with Section 3.3.1. A closed transition star-delta connection in accordance with
Section 3.3.4 is possible in both cases. With very large load-torques it may even be necessary.
The transition resistor an
d the transition contactor should be rated as described there.
The rules in accordance with Section 3.3.2 apply for the connections.
3.4 Auto-transformer starting
3.4.1 Circuit and function
An auto-transformer starter makes it possible to start squirrel-cage induction motors with
reduced starting current, as the voltage across the motor is reduced during starting. In contrast
to the star-delta connection, only three motor leads and terminals are required.
On starting, the motor is connected to the tappings of the auto transformer; transformer
contactor K2M and star contactor K1M are closed. The motor starts at the voltage reduced by
the transformer, with a correspondingly smaller current.
By this means the feeding current in comparison to direct starting would be reduced by the
square of the transformer voltage ratio; nevertheless, it
is in most cases noticeably higher, as it
also covers the relatively high transformer losses. Depending on the tapping and starting current
ratio of the motor, the starting current lies at (1 … 5) · I
e
. In contrast, the motor torque falls with
the square of the voltage across the windings. Auto-transformers usually have three available
taps in each phase (for example 80 %, 65 %, 50 %), so that the motor starting characteristic can
be adjusted to the load conditions.
LVSAM-WP001A-EN-P - April 2009
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